Perform a Pulse Chemisorption Analysis

Pulse Chemisorption Analysis Tutorial

This topic provides an example of Pulse chemisorption being performed on a sample of Pt/Al2O3 with CO. Make the appropriate modifications for the material being analyzed. Platinum Alumina Reference Material can be ordered from Micromeritics. Parts and accessories are located on the Micromeritics web page.

A Pulse chemisorption analysis determines the quantity of active gas irreversibly adsorbed. By applying pulses of known quantity of active gas to the sample, calculations of active surface area, percent metal dispersion, and active particle size can be made for supported metals. The sample is dosed with the analysis gas using the injection loop or a syringe until all accessible active sites are covered. The amount chemisorbed is the difference between the total amount of active gas injected and the amount that does not interact irreversibly with the sample.

The number of injections depends on the quantity in each pulse and total number of active sites present.

Loop Volume

Loop Volume

The gas quantity in each pulse is determined by active gas concentration, the loop or syringe volume, temperature, and pressure. The loop temperature is controlled by the instrument. The ambient (syringe) temperature and atmospheric pressure are entered by the operator. If a syringe is used, the volume injected is determined by the operator. In general, it is desirable for the sample to require at least two doses of gas, but no more than ten doses, before the reaction ends (although this may vary from lab to lab). Some factors that influence the number of doses required are sample size, the density of active sites, the concentration of active gas, and the size of the loop.

A 0.5 cm³ loop is provided with the analyzer. There are also 1 cm³ and 5 cm³ loops available.

Loop Calibration

For the most accurate determination of the injected gas quantity, a loop calibration experiment should be performed. An independent loop calibration can be performed before or after the analysis.

The following example assumes that the loop calibration will be performed after the analysis. A Loop Calibration step can be included in a pulse chemisorption analysis.

Preparation
Pretreatment	Degas by flowing inert gas (such as helium, argon, or nitrogen) over the sample while ramping the temperature. Hydrogen of at least 10% concentration is generally used for reduction.
Analysis	Pulse the loop gas over the sample until the peak area remains constant.
Sorption or Cold Trap	The trap does not need to be cooled during the reference material analysis. This may vary depending on application or analysis method. The trap tube should remain on the instrument. Including the trap in the flow path is optional.
Pressure regulator	Gas cylinders should be set to a level between 14 and 16 psig (95 and 110 kPag).
Furnace temperature	Select a temperature high enough to remove any contaminants or moisture, but not so high as to cause sintering or fusing of the sample. Ensure the Termination step is set to return the sample temperature to ambient.

Before performing an analysis, ensure the sample and analyzer are adequately prepared. See Prepare for Analysis.

Procedure

1. Obtain the sample mass, then install the loaded sample tube and thermocouple on the analyzer. Raise the furnace around the sample tube and install the insulator disc and shield.
2. Create a new sample file for this analysis. In the Analysis Conditions drop-down box, select either Pt-Al CO Pulse Chemisorption Loop or Pt-Al CO Pulse Chemisorption Syringe and modify the steps as needed depending on the analysis to be performed.
3. On the Report Options tab, select the required report options for the analysis.
4. Save and close the file.
5. Go to Unit [n] > Sample Analysis and select the sample file saved in the previous step.
6. Click Start to start the analysis.
7. When prompted, select the calibrations associated with each experiment in the sample file (if applicable). For this example, select None. Calibration files can also be associated with a sample file after analysis using the Peak Editor. See Peak Editor .

Use the cotton gloves provided in the accessory kit when handling heated surfaces. These cotton gloves are not intended to protect hands when heated surfaces are above 60 °C.

Generate the Report

Open the Peak Editor and ensure that peaks are properly marked.

There are three possibilities for each dose of gas injected during:

• all of the gas is taken up by the sample,
• some of the gas is taken up by the sample, or
• none of the gas is taken up by the sample.

When Pulse chemisorption is properly performed, there will be some injections of each type. When the data is viewed using the Peak Editor, however, only those injections in which some or none of the gas is taken up will appear as peaks. When all of the gas is taken up by the sample, none of it reaches the detector and, therefore, the peak area is zero. These types of peaks are detected automatically by the application and do not require marking.

To generate the report, go to Reports > Start Report> [.SMP file created above] or click Preview and select the signal view.